The territory government on Saturday strengthened travel restrictions to help stop the spread of COVID-19 as parts of NSW were plunged back into lockdown.

People who have arrived in the ACT from Greater Sydney, the Blue Mountains, the Central Coast or Wollongong since June 21 are now subject to "stay-at-home" requirements until July 9.

The same applies to returning residents who have not yet made it back.

Non-ACT residents who have recently been to the affected areas must not enter the territory without an exemption.

Acting chief police officer Liz McDonald said her people would be "on the lookout for vehicles from interstate".

"If you enter the ACT against the public health directions you may be stopped by police anytime, anywhere," Commander McDonald said late on Saturday.

"We will also continue having officers conducting checks of individuals subject to stay-at-home orders.

"If you suspect someone is breaching the public health directions, let us know.

"We rely on information from our community, and will act on information we receive and actively engage anyone who could be from the declared areas."

ACT Health Minister Rachel Stephen-Smith had earlier told reporters it would be "virtually impossible" to close the territory's border.

"So, while we will be putting in place spot checks, the border will not be closed to NSW residents and that means that situations in Sydney and situations in NSW present an extra risk to the ACT," she said.

People wanting to report non-compliance with public health directions can call the ACT COVID-19 helpline on 6207 7244.

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